In general, a battery charger is so designed that it charges a battery by supplying it with an electric current while the voltage across the battery is lower than a predetermined level, and that it thereafter stops charging when the voltage across the battery reaches that predetermined level as the result of the charging. FIG. 7 is a block diagram illustrating how a conventional battery charger is connected to a power supply device (hereafter referred to as the "battery pack"). In FIG. 7, numeral 60 represents the charger, and numeral 61 represents the battery pack that is charged thereby. When the voltage across the battery pack 61 is lower than a predetermined level, the charger 60 charges the battery pack 61 by supplying it with a current Ia, and meanwhile it keeps an LED (light-emitting diode) 64 on to indicate that charging is in progress (this LED will hereafter be referred to as the "charge-in-progress LED"). When the voltage across the battery pack 61 reaches the above-mentioned predetermined level, the charger 60 stops the supply of the current Ia and turns off the charge-in-progress LED 64, and in addition it turns on an LED 65 to indicate that charging has been completed (this LED will hereafter be referred to as the "charge-complete LED").
For example, in a case where the battery pack 61 is a power supply device that employs lithium-ion cells, the battery pack 61 incorporates a protection circuit that serves to secure stable operation of those cells by protecting them against overdischarge and other hazards. As a result, when overdischarge or the like is detected, this protection circuit inhibits the discharging of the battery pack 61 from within it. In this state, where both charging and discharging are inhibited, the charging of the battery pack 61 cannot be restarted without first canceling this inhibiting state. For this reason, the charger 60 is so designed that, even when it is not connected to the battery pack 61, it outputs a voltage almost as high as the voltage across the battery pack 61 in its fully charged state, and on the other hand the battery pack 61 is so designed that it cancels the above-mentioned inhibiting state by detecting a feeble current that flows into it when it is connected to the charger 60 and receives therefrom the above-mentioned high voltage.
At this time, however, precisely because the charger 60 is designed to output a high voltage on its current-supplying side, the charger 60, even when it is not connected to the battery pack 61, keeps the charge-complete LED 65 on, falsely indicating that charging has been completed. To overcome this inconvenience, the conventional charger 60 is, as shown in FIG. 7, fitted with a mechanical switch 63 for checking whether the battery pack 61 is present or not, so that, when it is not connected to the battery pack 61, it can keep both the charge-in-progress LED 64 and the charge-complete LED 65 off. Here, the mechanical switch 63 is a switch that has a contact that is mechanically opened and closed depending on whether the battery pack 61 is present or not.
Having such a mechanical switch 63, the conventional charger 60 is prone to malfunction due to imperfect mechanical contact in the mechanical switch 63, and thus it does not offer satisfactory safety. In addition, the use of the mechanical switch 63 increases the cost of the charger 60.